1. Field of the Invention
This invention relates to a printing apparatus and, in particular, to a printing apparatus in which the printing medium is firmly retained on the drum to enable high-accuracy feed without involving any positional deviation, and which is suitable for use as a thermal-transfer printing apparatus.
2. Description of the Prior Art
A feed mechanism in a conventional thermal-transfer printing apparatus is disclosed in Japanese Utility Model Laid-Open No. 62-48547, wherein a printing medium fastening gripper, provided on the outer periphery of a platen drum having a circumference greater than the length of the printing medium used, fastens the tip of the printing medium to the drum. The mechanism design is such that, each time this gripper passes the thermal printing head, the printing head is retracted so as to avoid colliding with the gripper. According to another conventional printing apparatus disclosed in Japanese Patent Laid-Open No. 62-94363, a clamping mechanism is provided within the platen drum, thereby securing the printing medium in position.
In still another conventional printing apparatus disclosed in Japanese Patent Laid-Open No. 61-137458, the edge of a printing medium is fastened by grippers at three positions on a gripper chain which guides them along a circular locus, thereby effecting rotational guidance of the printing medium between the platen roller and the printing head.
In the above-mentioned printing medium fastening mechanism using a gripper (Japanese Utility Model Laid-Open No. 62-48547), the thermal printing head must be retracted each time the gripper section passes to avoid colliding with the thermal printing head. The clamp mechanism disclosed in Japanese Patent Laid-Open No. 62-94363 involves a high drum cost and a complicated structure since the mechanism is lodged within the drum. Moreover, both conventional systems are so designed that the printing medium is only fastened at its tip, so that deviation of the printing medium during printing cannot be avoided completely, this being a hindrance to realizing a highly accurate feed function.
In the system for fastening the printing medium edge disclosed in Japanese Patent Laid-Open No. 61-137458, grippers can only be provided at several positions. That is, it only provides partial fastening and this does not retain the printing medium in a satisfactory manner, so that highly accurate feeding cannot be effected. Also, since the platen roller has to be shifted each time these grippers pass its position, a complicated mechanism is inevitable.